EV Motor by ZF Eliminates Rare Earth Magnets
An electric vehicle motor developed by ZF operates without permanent magnets made from rare earth materials.
The permanent-magnet synchronous motors (PSM) found in most electric vehicles use extremely powerful rare earth magnets in the rotor that move in response to an electromagnetic field created by the flow of electric current through the motor’s stator coils.
Permanent magnets contain rare earth elements. Image used courtesy of Adobe Stock
European automotive transmission manufacturer ZF has developed an electric traction motor for EVs that does not use permanent magnets. In the ZF motor, inductive currents are generated inside the rotor, enabling the ultra-compact motor to deliver performance data on par with permanent-magnet synchronous traction motors.
ZF’s magnet-free electric motor. Image used courtesy of ZF
Rare Earth Elements
Rare earth elements are 17 metallic elements (primarily the lanthanides) that are not particularly rare but are difficult to process. They are used in applications such as magnets, lasers, glass, and electronic components. From the 1960s through the 1980s, the U.S. was the largest producer of rare earths from the Mountain Pass Mine in California.
Mountain Pass Mine rare elements mine in California. Image used courtesy of USGS
Mountain Pass, which had closed in 2002, reopened in 2017 thanks to numerous government grants. Meanwhile, China has become the dominant player in rarer earths, supplying 80 percent of the market in 2020. The U.S. and European nations, mindful of the security concerns of China’s monopoly, are working to find other sources for the materials.
China’s share of the rare earth market declined to 60 percent of the total in 2021. However, China still processes 85 percent of the world's rare earths and produces 92 percent of the world's rare earth magnets. Removing these magnets from EV traction motors will help reduce the dependence on China—a long-term security concern both in the U.S. and in Europe.
Creating Current Without Magnets
Separately excited synchronous motors (SESM), where the magnetic field in the rotor is built up by the flow of an electric current instead of using rare earth permanent magnets, already exist. Conventional SESMs require sliding brushes to provide electric current to the coils in the rotor, which are prone to wear and take up space resulting in a larger motor.
The ZF I2SM (In-Rotor Inductive-Excited Synchronous Motor) does not use brush elements to send an electric current into the rotor. Instead, energy is transferred to the rotor through an inductive current that is set up via an inductive exciter consisting of coils within the rotor. The new ZF design without brushes requires up to 90 millimeters less space axially while also producing power and torque levels consistent with those produced by PSM traction motors found in EVs.
According to ZF, when compared to other common SESM designs, the inductive exciter used in the I2SM design can reduce energy transmission loss into the rotor by 15 percent. The company also notes that the carbon footprint in production can be up to 50 percent less than PSM traction motors with rare earth magnets. Mining and processing of rare earths can create large amounts of CO2 emissions.
ZF will develop the I2SM technology and provide it as an option to customers within its own e-drive platform. It will be engineered in 400 V and 800 V architectures for passenger and commercial vehicle applications, with the 800 V systems using silicon carbide chips in its power electronics.
